gitignore: add intermediates from libfdt build

Since ee95d10b (fdt: Build the new python libfdt module), a number of
additional files are auto-generated/installed into the tools directory.
List these in .gitignore to suppress having them listed as untracked.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
References: ee95d10b (fdt: Build the new python libfdt module)
Reviewed-by: Simon Glass <sjg@chromium.org>